1. Field of the Invention
The present invention relates to balanced SAW filters having a balanced-unbalanced converting function. More specifically, the present invention relates to a balanced SAW filter having first and second 5-IDT-type longitudinally-coupled-resonator SAW filter elements arranged in two stages that are cascade-connected to each other.
2. Description of the Related Art
In mobile communication devices, such as portable telephones, surface acoustic wave (SAW) filters are widely used as bandpass filters. In particular, as portable telephones are being reduced in size and becoming increasingly sophisticated, a so-called balanced SAW filter having a balanced-unbalanced converting function has been used as an RF-stage band pass filter that is connected to an antenna terminal. An example of a balanced SAW filter of this type is disclosed in Japanese Unexamined Patent Application Publication No. 2000-91883 (Patent Document 1).
Referring to FIG. 3, a balanced SAW filter 100 disclosed in Patent Document 1 includes first and second longitudinally-coupled-resonator SAW filter elements 101 and 102 that are cascade-connected to each other. The first longitudinally-coupled-resonator SAW filter element 101 includes first to fifth IDTs 111 to 115 that are arranged in a propagation direction of surface acoustic waves. At the opposite ends of the region in which the IDTs 111 to 115 are arranged in the propagation direction of surface acoustic waves, reflectors 116 and 117 are provided.
Similarly, the second longitudinally-coupled-resonator SAW filter element 102 includes sixth to tenth IDTs 121 to 125 that are arranged in the propagation direction of surface acoustic waves, and also includes reflectors 126 and 127.
First ends of the second and fourth IDTs 112 and 114 are mutually connected to each other so as to be connected to an unbalanced terminal 131. First ends of the first, third, and fifth IDTs are mutually connected to each other. A second end of the first IDT 111 is connected to the sixth IDT 121 by a first wiring line 141. Similarly, the third and fifth IDTs 113 and 115 are respectively connected to the eighth and tenth IDTs 123 and 125 by second and third wiring lines 142 and 143. On the other hand, the first end of the seventh IDT 122 is connected to a first balanced terminal 132. A second end of the IDT 122 is mutually connected to a first end of the IDT 124. A second end of the IDT 124 is connected to a second balanced terminal 133. Due to the configuration described above, the SAW filter 100 according to Patent Document 1 has a reduced size and produces less insertion loss.
Although the 5-IDT-type longitudinally-coupled-resonator SAW filter elements 101 and 102 are cascade-connected to each other in the balanced SAW filter 100, signals of the same phase flow through the first to third wiring lines 141 to 143.
On the other hand, a wiring line 144 that electrically connects the second balanced terminal 133 to the ninth IDT 124 extends outward from between the second and third wiring lines 142 and 143 by crossing over a region in which the first and second wiring lines 141 and 142 are disposed, as shown in FIG. 3. Moreover, a wiring line 145 that connects the first balanced terminal 132 to the seventh IDT 122 is disposed in a region distant from the region where the first to third wiring lines 141 to 143 are disposed.
Consequently, direct waves caused by stray capacitance between the first to third wiring lines 141 to 143 and the first and second balanced terminals 132 and 133 are large, and these large direct waves are unfavorably applied to the balanced terminals 132 and 133. In addition, the magnitude of the direct wave applied to the balanced terminal 133 is significantly greater than the magnitude of the direct wave applied to the balanced terminal 132. This is problematic in that the degree of balance between the balanced terminals 132 and 133 is deteriorated.